Differential input amplifiers, for example, operational amplifiers, are basic building blocks of many electrical circuits. A differential input amplifier basically has two otherwise balanced current paths coupled to a constant current source. The amplifier output voltage is generated by application of input signals that alter the respective current through the current paths.
In many applications, it is desirable to have a differential input amplifier operating with a predefined quiescent output voltage (i.e., a predefined output voltage when there is no potential difference between its inverting and non-inverting inputs). However, because of the presence of component mismatches in the constitution of the two current paths, the quiescent output voltage of most differential amplifiers usually departs from the predefined value.
This departure presents many problems to circuit designers. For example, when an operational amplifier is used in an integrator circuit, the quiescent output voltage departure would result in a time-dependent component in the integrator output. In other applications, a quiescent output voltage departure also places a lower limit on the magnitude of the DC (direct current) voltage which can be accurately amplified by the amplifier.
U.S. Pat. No. 4,068,182 is a prior art attempt to provide a differential input amplifier with adjustable quiescent output voltage. Unfortunately, adjustment provided by that invention is good for a particular configuration (i.e. p-channel differential inputs). For n-channel differential inputs, adjustment of the amplifier must reference to the most positive potential of the circuit, which is usually V.sub.DD. This limitation is bad since in very large scale integrated (VLSI) circuits, there usually exists a large amount of digital logic switching that may cause wide fluctuations in the V.sub.DD line. As a result, accurate adjustment in this configuration is usually not possible. Moreover, for p-channel differential inputs, quiescent output voltage adjustment requires an unbalanced current mirror amplifier (CMA), which in turn would introduce non-symmetrical slew rate.
Therefore, there is a need for a differential input amplifier circuit with provision for adjustment of its quiescent output voltage.